Two component polyurethane adhesive compositions generally comprise components that are liquid or pastes at room temperature before they are mixed together. The first component of the composition comprises a polyol and other ingredients, such as chain extenders, catalysts, blocking agents and other additives as desired. The second component comprises monomeric, polymeric or prepolymer isocyanate. In order to make a bond, the two components of the adhesive are fully mixed together and the composition is then applied to a substrate. The mixed composition then initiates cure and develops bonding strength while transforming into a solid form. The curing reaction takes place between the free isocyanate groups and the active hydrogens from the polyol. If there are excess free isocyanate groups after the main curing reaction, the excess free isocyanate groups are cured by ambient or surface moisture from the substrates.
Two component polyurethane adhesives offer many advantages over single component compositions. One such advantage is that two component adhesives have extremely long shelf lives. Further, such adhesives may be formulated with a wide range of different components. The wide range of components provides end users with great latitude to design different formulations that will meet various end use application requirements.
In formulating two part adhesive compositions, there is often a trade off between a desired rapid cure rate and a desired long open time. The long open time is desirable because it enhances process flexibility and provides good wet out to the substrates to be bonded. The addition of catalysts generally increases the cure speed but also reduces the open time. There is a need in the art for a two part adhesive composition that will provide an induction period between the mixing of the components and the initiation of a rapid cure. The induction period would allow for long open time and good wet out of the substrates. The induction period would also allow for the maximum work life period for the adhesive which would in turn simplify the design of processing equipment. Upon initiation of curing, the rapid cure would provide as fast a process as possible to minimize the processing time.